add kernel scheduler service on background isolate

Tiered Timer.periodic instances (1min, 10min, 15min, 1hr, midnight)
running on a dedicated Dart isolate. Tick messages sent back to the
main isolate via SendPort and emitted as SchedulerTick events on the
DaemonBus. Extensions subscribe with events.on<SchedulerTick>() and
filter by tier.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/kernel.dart

@@ -34,6 +34,7 @@ export 'src/notify.dart';
 export 'src/os.dart';
 export 'src/panels/arrangement.dart';
 export 'src/project.dart';
+export 'src/scheduler.dart';
 export 'src/secrets.dart';
 export 'src/tray.dart';
 export 'src/panels/drag_resize.dart';